NLTT 43806 is a white dwarf, the dying remnants of a star like our own. But there's a problem. Its composition doesn't make any sense...unless there was a cataclysm just like the one Earth experienced four billion years ago.

Located about fifty light-years away, NLTT 43806 is unlike any other white dwarf we've seen before. Its aluminum content is abnormally high relative to the amount of iron. On all other white dwarfs we've seen, iron generally outnumbers aluminum by a factor of about ten to one, due to the vagaries of the chemical reactions that go on within the star.

Where you can find a lot of aluminum is in the crust of rocky planets like Earth. After oxygen and silicon, aluminum is the most abundant element in Earth's crust. If you want to find iron, you need https://publish.io9.com/ged/new#to go in deeper, as the core is where most of our planet's iron is found.

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And it isn't just iron and aluminum - a total of nine elements identified on the white dwarf are found in abundance levels that don't make much sense for a star, but fit neatly with what you'd expect on the top layers of a rocky planet. This suggests that NLTT 43806 has at least one rocky planet...but it probably once had an extra. These two planets collided into each other, ripping off huge portions of the crust and upper mantle of the planets.

That collision probably was a lot like the one between Earth and a Mars-sized object, commonly known as Theia. This ancient impact - which we've been talking abouta lot lately - was responsible for creating Earth's Moon. So then, NLTT 43806 might not just have a planet - it might also have a large, newly formed moon in orbit around that planet.

The two sides of the Moon look nothing like each other: the near side is flat and low, while the…
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And we do mean newly formed - since the unusual elements are still detectable at all on the white dwarf, that means they're still near the surface. Such elements would be pulled down beneath the surface fairly quickly, so that means the collision must still be raining down material on the star. That places the impact at no more than 50 million years ago, which is practically no time at all in cosmic terms.

It's pretty cool to think that we've detected a recent example of arguably the most important event in our planet's history, but it's not the only possible explanation - it's also conceivable that a large asteroid crashed onto the white dwarf, depositing these unusual elements.

It appears unlikely with our present technology that we will be able to detect the presence of a planet around NLTT 43806 - the circumstances just aren't lined up right for that - but we may be able to confirm or refute the hypothesis by studying the levels of manganese and potassium on the surface, which would look different depending on the precise source of these elements.